Angular position is typically measured with respect to one or more of three axes that are known as the roll-axis, the yaw-axis, and the pitch-axis. These three axes may be illustrated by reference to a pilot sitting in a nose compartment of an aircraft, it being understood that angular measurements are useful in a wide variety of situations not limited to piloting an aircraft. The roll-axis extends longitudinally through the nose and tail of the aircraft, passing through the pilot from front to back. The yaw-axis, which is substantially perpendicular to the roll-axis, extends vertically through the floor and ceiling of the pilot's compartment. The pitch-axis, which is substantially perpendicular to the other two axes, extends horizontally through the left and right side walls of the pilot's compartment.
Changes in relative angular position result from rotation about one or more of these three axes. For instance, if the craft as initially described is in level flight, a one-quarter turn about the pitch-axis will move the craft to a vertical position in which the craft is headed either straight up or straight down, depending on the direction of the turn.
Accurate measurements of relative angular position are useful in a variety of scientific, technical, and industrial applications. For instance, tooling machines typically contain a blade or other cutting element whose position relative to the workpiece is critical. Angular position measurements are used to measure the relative position of the blade and the workpiece in both automated and hand-controlled tooling machines. Accurate angular measurements are also important in bore-sighting alignment and assembling aircraft, rockets, and other aerospace vehicles. The angular position of the fuselage relative to extensions such as the wings, fins, and tail is often critical. Thus, there is a need for instruments which accurately measure the angular position of one structure relative to another structure.
One conventional approach to measuring relative angular position about a roll-axis includes the use of an incandescent lamp which shines two light beams through a rotating disk-shaped linear polarizer. The center of the disk-shaped polarizer is secured to a rotating shaft of an electric motor. The motor is driven at a constant angular rate, thereby modulating the light traveling through an annular portion of the polarizer with respect to time.
The polarizers are generally positioned such that the roll-axis extends generally between the rotating polarizer and a movable target linear polarizer. One of the light beams, known as the "target beam," travels along a first path from the lamp through the rotating polarizer and the target polarizer to a target photosensor. The target photosensor produces a target signal corresponding to the received intensity of the target beam over time.
Another light beam, known as the "reference beam," travels along a different path. The reference beam travels from the lamp through the rotating polarizer, through a fixed reference linear polarizer, and then to a reference photosensor. The reference photosensor produces a reference signal corresponding to the received intensity of the reference beam over time. The phase difference between the reference signal and the target signal corresponds generally to the relative angular position of the reference polarizer and the target polarizer about the roll-axis. Thus, by measuring this phase difference, the target's angular position about the roll-axis is determined.
However, such an approach is sensitive to imperfections in the rotating polarizer. The beam directed at the target photosensor and the beam directed at the reference photosensor pass through different annular portions of the rotating polarizer. Thus, imperfections in one or both annular portions of the rotating polarizer may introduce noise into the reference signal relative to the target signal. Polarizer imperfections may also introduce noise into the target signal alone or the reference signal alone.
In addition, the reference signal is produced by electronically converting the intensity of the polarized reference beam at the reference photosensor into an analog intensity signal and then into a sine wave reference signal. Electronics used to convert periodically increasing and decreasing light intensity into a reference signal typically introduce a phase shift for which the system must compensate. The electronics may also introduce noise during the conversion.
Another conventional approach to measuring relative angular position about a roll-axis includes the use of a laser which emits a single beam of light that is polarized by an annular portion of a rotating linear polarizer. The polarized beam travels through a beam splitter having a mirrored side. A reference portion of the beam is directed by the beam splitter to a fixed reference linear polarizer, and then to a reference photosensor. The reference photosensor produces a reference signal corresponding to the reference beam intensity over time.
A target portion of the beam passes through the beam splitter to a target polarizer and a target retro-reflector. The target retro-reflector reflects the target beam back to the mirrored side of the beam splitter, which in turn directs the target beam to a target photosensor. The target photosensor produces a target signal corresponding to the target beam intensity over time. The phase difference between the reference signal and the target signal corresponds generally to the relative angular position of the reference polarizer and the target polarizer about the roll-axis.
This approach sends only one beam through the rotating polarizer. Thus, optical imperfections in the rotating polarizer are much less likely to introduce noise than in conventional approaches that send two beams through a rotating polarizer. However, the beam still travels through an annular region offset from the center of the rotating polarizer, so polarizer imperfections may still introduce noise into the target signal and into the reference signal.
In addition, the reference signal is still created by converting polarized light intensity into a reference signal. Thus, the problems of phase shifting and noise noted above also arise in connection with the conversion electronics of the beam-splitter approach.
Moreover, it is often desirable to measure pitch and yaw as well as roll. However, the target retro-reflector is designed to always return the beam at the same angle. The beam must be aligned to return to the beam splitter. Changes in the target's pitch or yaw relative to the beam splitter are not captured by the beam, so the beam-splitter approach is restricted to roll measurements.
Thus, it would be an advancement in the art to provide a system for measuring angular position about a roll-axis which is insensitive to imperfections in the annular portions of a rotating polarizer.
It would also be an advancement to provide such a system which does not rely on the electronic conversion of polarized light intensity to produce a reference signal.
It would be a further advancement to provide such a system which also accurately measures changes in position about a yaw-axis and a pitch-axis.
Such a system for measuring angular position is disclosed and claimed herein.